Often we have data on units at multiple points in time--that’s called panel data. We introduce the main approach to using panel data to learn about causality in this video. We illustrate this idea by looking at the effect of the death penalty on homicide rates.
The paper is Manski and Pepper (2013). "Deterrence and the Death Penalty: Partial Identification Analysis Using Repeated Cross Sections.'' Journal of Quantitative Criminology, 29(1), 123-141.